1. Field of the Invention
This invention relates to a circuit breaker system comprising a plurality of modular circuit breakers which are connected in paralleled branches of a main conductor to protect equipment carrying currents greater than the current rating of the individual circuit breakers. The invention includes modular breakers which are responsive to overcurrents in the individual branch protected by the modular breaker in addition to any overcurrent in the main conductor. The invention further relates to auctioneering the branch current and a current representative of the total current in the main conductor and to reducing the burdens on the sources of such auctioneered currents.
2. Background Information
Typically, an electric distribution system is protected by a main circuit breaker to disconnect the entire distribution system from the source, and a hierarchy of progressively smaller breakers protecting the various branches of the system. The circuit breakers at the source end of the system are designed to withstand overcurrents for a period of time to allow the branch circuit breakers to operate and hopefully isolate the fault without shutting down the entire system. Thus, the main breaker must be able to carry large currents indefinitely and withstand even higher currents for short periods of time. Such breakers are expensive to build and especially so since the market for such large units is small.
In addition, if the main breaker is out of service, the entire distribution system is inoperative. Another drawback of such large main breakers is their size. Not only are they heavy to move, but they can be difficult to remove for replacement. For instance, in a submarine, the main breaker will not fit through the hatch.
There have been applications in which medium voltage breakers have been paralleled to protect equipment from large currents. In these prior art installations, the paralleled breakers are controlled solely by a control signal representative of the total current to be interrupted by the breakers. No protection is provided for the current carried by the individual breakers. This can result in overloading of an individual breaker, since it is very difficult to evenly distribute the total current between the paralleled breakers. In such systems, equalization of the currents is attempted by passive means such as matching as closely as possible the physical parameters of the paralleled circuits incorporating the individual breakers, or by active means such as by using balancing current transformers. The passive techniques achieve limited success in balancing the currents and the active methods add cost and complexity to the system.
It is known to auctioneer direct currents to select the largest current. One technique for auctioneering dc currents is disclosed in U.S. Pat. No. 3,689,801 where series connected dc current sources are each shunted by a diode poled to be forward biased by the other current sources. Ac currents are auctioneered to select the largest current by connecting in series the dc terminals of full wave rectifier bridges which convert the ac signals to unidirectional signals. U.S. Pat. Nos. 3,852,660 and 3,924,160 disclose circuit breakers in which the phase currents of a three phase power line are auctioneered using series connected full wave rectifier bridge circuits to trip the breaker on the phase current having the greatest instantaneous magnitude.
Despite these developments, There remains a need for a system in which circuit breakers can be paralleled and respond to overloads in total current while also being protected against overloads in the current flowing through the breaker.
There is also a need for such a circuit breaker system which is modular such that individual units can be selectively removed or replaced.
There is an additional need for such a circuit breaker system in which the individual breakers are responsive to the largest of the total and branch phase currents.